Test layer for Mercurial.
Migrate the synchorize test to use layers

- Still need to keep the ztc.FunctionalTestCase as the base class due to
  the need to "mimic" http access to the local site to avoid the server
  from being blocked from accessing itself during the tests.
- Drop the test level since this should no longer cause problems, even
  though it seems like the server thread can be properly killed now.